It is conventionally known a surface processing (blasting, shot peening, coating, etc.) of a workpiece by ejecting “abrasive grain,” “shot,” “granular material such as coating material” or the like as ejection material together with compressed air directed onto the workpiece. Known nozzle assemblies for such surface processing of workpieces include a pressure-type nozzle assembly (Patent Literature 1) in which a pressure vessel filled with ejection material is pressurized by compressed air or the like and ejection material is fed to a pressurized air flow and ejected as a solid-gas two phase flow together with the pressurized air; and a suction-type nozzle assembly (Patent Literature 2) in which ejection material is suctioned in by a suction force from a compressed air flow introduced into an air nozzle and ejected as a solid-gas two phase flow together with the compressed air.
Compared to a suction-type nozzle assembly, a pressure-type nozzle assembly is able to eject ejection material at a high velocity, so it has a high processing capacity; it is able to impart compressive residual stress down to a lower depth position in a processing of shot peening, and is able to form a high adhesion strength coating during the coating process. However, the pressure-type nozzle assembly has the problem that ejection material volume and ejection time is limited by the capacity of the pressurizing tank, so that long duration continuous ejection is not possible.
In contrast, a suction-type nozzle assembly has an advantage that the container for holding ejection material does not need to be xsealed, therefore the ejection material can be continuously ejected. However a suction-type nozzle assembly suctions ejection material exposed to the atmosphere together with air, leading to the problem of pressure loss, which slows the ejecting velocity compared to a pressure-type nozzle. A conceivable way to increase an ejecting velocity is to compensate for the compressed air pressure loss portion by raising the pressure of the compressed air. However this requires a stronger compressed air source, which leads to increased equipment costs and operating energy for the compressed air source, etc.